Structural effect of imidazolium-type ionic liquid adsorption to montmorillonite

The adsorption of 1-alkyl-3-methylimidazolium-type ionic liquids (ITILs) coupled with different counteranions (Tf₂N^-, PF₆^-, BF₄^-, and Cl^-) with variational cation alkyl chain lengths (n = 2, 4, 6, and 8) to montmorillonite was investigated to explore the structural effect of ITILs on their adsorption. A series of montmorillonite with different cation exchange capacities (CECs) and possessing a set of homoionic K- and Cs-exchanged interlayer cations were also examined to assess the influence of montmorillonite structure and characteristics. The adsorption of ITILs to Na-saturated montmorillonite (Na-MAz) was counteranion-independent but increased with the increase in the alkyl chain length of the imidazolium cation. X-ray diffraction results indicated that ITIL cations with different alkyl chains lay flat between the montmorillonite interlayers with different contact angles. The uptake of ITILs by Na-MAz increased with the increase in the solution pH and decrease in ionic strength. Na-MAz exhibited greater adsorption than K- and Cs-saturated MAz due to the larger hydrated radii of Na⁺ than those of K⁺ and Cs⁺. The uptake of ITILs to Na-MZj (CEC = 64 mmol/100 g) was almost half compared with that of Na-MAz (CEC = 117 mmol/100 g). Consequently, this work demonstrated that the ITIL adsorption to montmorillonite was dependent on the structures of both adsorbate and adsorbent.